Photon induced formation of metal comprising elongated nanostructures

ABSTRACT

The preferred embodiments provide a method for forming at least one metal comprising elongated nanostructure on a substrate. The method comprises exposing a metal halide compound surface to a photon comprising ambient to initiate formation of the at least one metal comprising elongated nanostructure. The preferred embodiments also provide metal comprising elongated nanostructures obtained by the method according to preferred embodiments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 60/948,129 filed on Jul. 5, 2007, U.S.Provisional Application No. 60/970,844 filed on Sep. 7, 2007, and U.S.Provisional Application No. 61/050,848 filed on May 6, 2008, and claimsthe benefit under 35 U.S.C. §119(a)-(d) of European Application No.07075949.3 filed on Nov. 1, 2007, the disclosures of which are herebyexpressly incorporated by reference in their entirety and are herebyexpressly made a portion of this application.

FIELD OF THE INVENTION

Elongated nanostructures such as nanowires are provided. Moreparticularly, a method is provided for forming at least one elongatednanostructure which comprises metal on a substrate and to elongatednanostructures which comprise metal thus obtained.

BACKGROUND OF THE INVENTION

In the field of molecular nano-electronics, semiconductor nano-crystals,nanowires (NWs) and carbon nanotubes (CNTs) are becoming more and moreimportant as components for various electronic devices. These NWs andCNTs are unique for their size, shape and physical properties and have,depending on their electrical characteristics, been used in electronicdevices such as e.g. diodes and transistors. Although a lot of progresshas been made on both fabrication and understanding of the limits ofperformance of these NWs and CNTs, there are still key issues to beaddressed for potential technological applications.

In the last years, a lot of effort has been put in the synthesis ofelongated nanostructures such as nanowires. Due to their restrictedsize, these structures exhibit novel physical and chemical propertiesand have opened up a large new field of basic research as well aspossible applications. Metallic nanowires show high potential for beingused in a wide range of advanced applications. Copper, for example, isof particular interest because of its high electrical conductivity. Withrapid shrinking in size of electronic devices, copper nanowires may playan essential role to form interconnects in devices in nano-electronicsand opto-electronics.

In Adv. Mater. 2001, 13(1), 62-65, Molares et al. report the fabricationof cylindrical poly- and single-crystalline copper wires by means of atemplate method. Copper wires with diameters between 60 nm and 500 nmand aspect ratios (ratio of length to diameter) up to 500 can beobtained. A disadvantage of the method may be that the use of a templateis required for obtaining free-standing wires.

The growth of CuO nanowires by heating Cu in an oxygen atmosphere hasbeen reported in the past (Xuchuan et al., Nano Letters, 2(12), 2002,pp. 1333-1338). However, the method described in this paper may berelatively slow, i.e. it may take several hours to grow the nanowires,and may require high temperatures, i.e. temperatures of up to 700° C.

Yong et al. reported reduction of CuO to form Cu nanowires in a plasma(Nanotechnology 18 (2007) 035608 (4 pp)). Aligned Cu nanowires wereprepared by the reduction of aligned CuO nanowires in electron cyclotronresonance (ECR) hydrogen plasma at room temperature.

CuO appears to be the only material that can be used presumably becauseproblems can occur if the heating step just results in evaporation fromthe surface instead of growing wires, e.g. for volatile compoundsstarting from about 150° C.

SUMMARY OF THE INVENTION

A good method is provided for forming at least one metal comprisingelongated nanostructure on a substrate and to metal comprising elongatednanostructures thus obtained.

With metal comprising elongated nanostructure is meant a nanostructurethat comprises metal.

The method according to preferred embodiments does not require the useof catalyst particles.

By using the method according to preferred embodiments elongatednanostructures can be formed which extend from the substrate in adirection, when the substrate is lying in a plane, substantiallyperpendicular to the plane of the substrate. Furthermore, by using themethod according to preferred embodiments free-standing metal comprisingelongated nanostructures can be formed.

The above advantages are provided by a method and device according tothe preferred embodiments.

In a first aspect, the preferred embodiments provide a method forforming at least one metal comprising elongated nanostructure onto asubstrate. The method comprises:

-   providing a substrate comprising at least a metal surface layer,-   converting at least part of the metal surface layer into a metal    halide compound, e.g. a metal halide compound different from a metal    fluoride compound,-   exposing the metal halide compound, e.g. a metal halide compound    different from a metal fluoride compound to a photon comprising    ambient to initiate formation of the at least one metal comprising    elongated nanostructure, and-   during exposure to the photon comprising ambient, volatile copper    halide products are formed and the concentration of the volatile    copper halide products in the reaction chamber is above the    saturation level of the volatile copper halide products to initiate    formation of at least one metal comprising elongated nanostructure.

It is an advantage of the method according to preferred embodiments thatit does not require provision of catalyst particles to form theelongated nanostructures. Deposition of catalyst nanoparticles is anadditional step in the process which may take some time. Providingcatalyst nanoparticles may be difficult to perform because the particlesneed to be deposited uniformly. Furthermore, if elongated nanostructureswith particular sizes, e.g. with a particular diameter, have to beformed, the size of the nanoparticles has to be chosen carefully as, ingeneral, the size of the nanoparticles determines the diameter of theelongated nanostructures grown from these nanoparticles.

According to preferred embodiments, the metal may be copper and theelongated nanostructure may be a copper comprising elongatednanostructure. According to these embodiments, providing a substratecomprising at least a metal surface layer may be performed by providinga substrate comprising at least a copper surface layer. Converting atleast part of the metal surface layer into a metal halide compound maybe performed by converting at least part of the copper surface layerinto a copper halide compound.

According to preferred embodiments, converting at least part of themetal surface layer into a metal halide compound may be performed byexposing at least part of the metal surface layer to a halogencomprising gas.

According to other preferred embodiments, converting at least part ofthe metal surface layer into a metal halide compound may be performed byexposing at least part of the metal surface layer to a halogencomprising plasma.

Exposing at least part of the metal surface layer to a halogencomprising plasma may be performed at 600 Watt and 10 mTorr during anexposure time period of 10 seconds.

The halogen comprising plasma may be a HBr or Cl₂ comprising plasma.

Exposing the metal halide compound to a photon comprising ambient may beperformed by exposing the metal halide compound to a He, Ar or Hcomprising plasma.

According to preferred embodiments, the metal halide compound may beCuCl_(x) and the photon comprising ambient may be a He plasma, andexposing the metal halide compound to a photon comprising ambient may beperformed at 1000 Watt and 30 mTorr without substrate bias during anexposure time period of at least 30 seconds, for example for a timeperiod of 120 seconds.

According to other preferred embodiments, the metal halide compound maybe CuBr_(x) and the photon comprising ambient may be a He plasma, andexposing the metal halide compound to a photon comprising ambient may beperformed at 1000 Watt and 80 mTorr without substrate bias during anexposure time period of at least 30 seconds, for example for a timeperiod of 120 seconds.

Providing a substrate comprising at least a metal surface layer may beperformed by:

-   providing a substrate, and-   providing a metal surface layer onto the substrate.

According to preferred embodiments, the method may furthermore comprise,before providing a metal surface layer, providing a protective layerand/or a barrier layer onto the substrate. The barrier layer may be usedto avoid diffusion of metal into the substrate. According to preferredembodiments, the method may comprise providing one of a protective layeror a barrier layer. According to other preferred embodiments, the methodmay comprise providing both a protective layer and a barrier layer.

The method may furthermore comprise, before converting at least part ofthe metal surface layer into a metal halide compound, providing apattern onto the metal surface layer.

Providing a pattern onto the metal surface layer may be performed by:

-   providing a layer of material, e.g. dielectric material, onto the    metal surface layer, and-   providing at least one hole in the layer of material.

According to preferred embodiments, the method may furthermore comprise,after formation of the metal comprising elongated nanostructure,removing the pattern.

According to still further embodiments, the method may furthermorecomprise, before exposing the metal halide compound to a photoncomprising ambient, cleaning the substrate.

Cleaning the substrate may be performed by a H₂ or N₂ plasma.

In a further aspect, the use is provided of the method according topreferred embodiments for forming copper comprising elongatednanostructures.

In still a further aspect, the preferred embodiments provides a metalcomprising elongated nanostructure comprising a combination of a metaland a metal halide compound, e.g. a metal halide compound different froma metal fluoride compound, the metal forming the core of the elongatednanostructure and the metal halide compound, e.g. a metal halidecompound different from a metal fluoride compound forming the shell ofthe elongated nanostructure, the shell surrounding the core.

The metal comprising elongated nanostructure may comprise between 60%and 99% metal and between 1% and 40% metal halide.

The metal comprising elongated nanostructure may comprise more than 95%metal and less than 5% metal halide.

The metal comprising elongated nanostructure may have a length between500 nm and 40 μm and a diameter between 50 nm and 200 nm.

According to preferred embodiments, the metal may be copper and themetal halide may be a copper halide.

According to preferred embodiments, the copper halide may be CuCl_(x) orCuBr_(x).

According to preferred embodiments, the metal comprising elongatednanostructure may be a metal comprising nanowire.

Particular and preferred aspects are set out in the accompanyingindependent and dependent claims. Features from the dependent claims maybe combined with features of the independent claims and with features ofother dependent claims as appropriate and not merely as explicitly setout in the claims.

Although there has been constant improvement, change and evolution ofdevices in this field, the present concepts are believed to representsubstantial new and novel improvements, including departures from priorpractices, resulting in the provision of more efficient, stable andreliable devices of this nature.

The above and other characteristics, features and advantages of thepreferred embodiments will become apparent from the following detaileddescription, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thisdescription is given for the sake of example only, without limiting thescope of the invention. The reference figures quoted below refer to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

All figures are intended to illustrate some aspects and preferredembodiments. The figures are depicted in a simplified way for reason ofclarity. Not all alternatives and options are shown and therefore theinvention is not limited to the content of the given drawings. Likenumerals are employed to reference like parts in the different figures.

FIGS. 1A to 1C illustrate subsequent processing steps in a methodaccording to preferred embodiments.

FIGS. 2A to 2E illustrate subsequent processing steps in a methodaccording to preferred embodiments.

FIGS. 3A to 3F schematically illustrate the influence of the initialthickness of the metal surface layer to formation of metal halidecompound and on the morphology and length of the nanostructures formed.

FIGS. 4A to 4C illustrate the influence of pressure in the plasmachamber during exposure of a CuCl_(x) layer to a He plasma on formationof nanostructures.

FIGS. 5A and 5B illustrate the influence of plasma power during exposureof a CuCl_(x) layer to a He plasma on formation of nanostructures.

FIG. 6 shows an AES survey scan recorded on a single Cu-NW grownstarting from a CuCl_(x) layer and on the substrate surface underneaththe Cu-NW.

FIG. 7A shows an AES survey scan recorded on a single Cu-NW grownstarting from a CuBr_(x) layer and FIG. 7B shows an AES survey scanrecorded on the substrate surface underneath.

FIGS. 8 to 11 illustrate formation of nanostructures according topreferred embodiments.

In the different figures, the same reference signs refer to the same oranalogous elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

Moreover, the term top and the like in the description and the claimsare used for descriptive purposes and not necessarily for describingrelative positions. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the preferredembodiments described herein are capable of operation in otherorientations than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepreferred embodiments, the only relevant components of the device are Aand B.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the preferred embodiments. Thus, appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment, but may. Furthermore, the particular features,structures or characteristics may be combined in any suitable manner, aswould be apparent to one of ordinary skill in the art from thisdisclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplarypreferred embodiments, various features of the invention are sometimesgrouped together in a single embodiment, figure, or description thereoffor the purpose of streamlining the disclosure and aiding in theunderstanding of one or more of the various inventive aspects. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed invention requires more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive aspects lie in less than all features of a singleforegoing disclosed embodiment. Thus, the claims following the detaileddescription are hereby expressly incorporated into this detaileddescription, with each claim standing on its own as a separateembodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that preferred embodiments may bepracticed without these specific details. In other instances, well-knownmethods, structures and techniques have not been shown in detail inorder not to obscure an understanding of this description.

The following terms are provided solely to aid in the understanding ofthe invention.

With the term “elongated nanostructures” is meant any two-dimensionallyconfined pieces of solid material in the form of wires (nanowires),tubes (nanotubes), rods (nanorods) and similar elongated substantiallycylindrical or polygonal nanostructures having a longitudinal axis. Across-dimension of the elongated nanostructures preferably lies in theregion of 1 to 500 nanometers.

The term “pattern” as referred to in the claims and the description areused to define structures (e.g. vias, trenches, and the like) in a layerdeposited onto a substrate e.g. a wafer substrate. The patterns areformed on the wafer substrates using patterning tools known by a personskilled in the art as a combination of lithography techniques(photolithography and e-beam lithography) and etching techniques (suchas reactive ion etching (RIE)).

The invention will now be described by a detailed description of severalpreferred embodiments. It is clear that other preferred embodiments canbe configured according to the knowledge of persons skilled in the artwithout departing from the true spirit or technical teaching of theinvention, the invention being limited only by the terms of the appendedclaims.

The preferred embodiments provides a method for forming at least onemetal comprising elongated nanostructure onto a substrate and to metalcomprising elongated nanostructures thus obtained. The method comprises:

-   providing a substrate comprising at least a metal surface layer,-   converting at least part of the metal surface layer into a metal    halide compound, e.g. a metal halide compound different from a metal    fluoride compound,-   exposing the metal halide compound, e.g. a metal halide compound    different from a metal fluoride compound to a photon comprising    ambient to initiate formation of the at least one metal comprising    elongated nanostructure, and-   during exposure to the photon comprising ambient, volatile copper    halide products are formed and the concentration of the volatile    copper halide products in the reaction chamber is above the    saturation level of the volatile copper halide products to initiate    formation of at least one metal comprising elongated nanostructure.    Throughout the description and the claims, with metal comprising    elongated nanostructure is meant a nanostructure that at least    comprises metal.

By using the method according to preferred embodiments, metal comprisingelongated nanostructure, e.g. copper comprising nanowires (Cu-NW), maygrow onto the substrate, more particularly may grow vertically onto thesubstrate. With growing vertically onto the substrate is meant that,when the substrate is lying in a plane, the metal comprising elongatednanostructures grow in a direction substantially perpendicular to theplane of the substrate. Furthermore, the metal comprising elongatednanostructures may be free-standing nanostructures. With free-standingnanostructures is meant that the nanostructures form sole entities onthe substrate and are not entangled with or do not contact neighbouringnanostructures.

The method according to preferred embodiments can be used to formelongated nanostructures comprising a combination of a metal and a metalhalide compound, such as e.g. MeCl_(x) and/or MeBr_(x), with Me being ametal. For example, the metal halide compound may be a metal halidecompound different from a metal fluoride compound. Metal comprisingelongated nanostructures formed by a method according to preferredembodiments comprise a combination of a metal and a metal halidecompound. The metal forms a core of the elongated nanostructure and themetal halide compound forms a shell of the elongated nanostructure, theshell substantially completely surrounding the core.

According to preferred embodiments, the elongated nanostructures formedand comprising a metal core and metal halide compound shell may befurther exposed to a halide comprising ambient so as to substantiallycompletely convert the metal of the core into a metal halide. In thatway, an elongated nanostructure completely formed of a metal halidecompound and comprising no pure metal anymore may be formed. Examplesinclude CuCl_(x) or CuBr_(x) nanowires. CuCl_(x) or CuBr_(x) nanowiresare semiconductors with large bandgaps (e.g. about 3.4 eV for CuCl_(x)and about 3 eV for CuBr_(x)), which makes them suitable to be used inoptoelectronic devices or for sensor applications.

According to other preferred embodiments, the metal halide compoundshell may be converted into metal by e.g. using a H₂ plasma. In thatway, pure metal elongated nanostructures may be formed. Pure metalelongated nanostructure may, for example, be used as interconnects.

Depending on the composition, the metal comprising elongatednanostructures formed by the method according to preferred embodimentscan show conducting or semiconducting properties.

An advantage of the method according to preferred embodiments is that itdoes not require the use of catalyst particles. Deposition of catalystnanoparticles is an additional step in the process which may take sometime. Providing catalyst nanoparticles may be difficult to performbecause the particles need to be deposited uniformly. Furthermore, ifelongated nanostructures with particular sizes, e.g. with a particulardiameter, have to be formed, the size of the nanoparticles has to bechosen carefully as, in general, the size of the nanoparticlesdetermines the diameter of the elongated nanostructures grown from thesenanoparticles.

Hereinafter, the method according to preferred embodiments will bedescribed by means of copper comprising nanowires (Cu-NWs). It has to beunderstood that this is not intended to limit the invention in any way.The method according to preferred embodiments may also be applied forforming other metal comprising nanowires, and for forming any metalcomprising elongated nanostructure.

FIGS. 1A to 1C illustrate subsequent processing steps in a methodaccording to preferred embodiments to form copper comprising nanowires(Cu-NWs) 6 on a substrate 1. It has to be understood that this is onlyby way of an example and is not intended to limit the invention in anyway. According to other embodiments, the method may furthermore compriseother steps, or may comprise a different sequence of steps.

According to the embodiment illustrated in FIGS. 1A to 1C, in a firststep a substrate 1 is provided. The substrate 1 can be any suitablesubstrate 1. In preferred embodiments, the term “substrate” may includeany underlying material or materials that may be used, or upon which adevice, a circuit or an epitaxial layer may be formed. In otheralternative embodiments, this “substrate” may include a semiconductorsubstrate such as e.g. doped silicon, a gallium arsenide (GaAs), agallium arsenide phosphide (GaAsP), an indium phosphide (InP), agermanium (Ge), or a silicon germanium (SiGe) substrate. The “substrate”may include for example, an insulating layer such as a SiO₂ or a Si₃N₄layer in addition to a semiconductor substrate portion. Thus, the termsubstrate also includes silicon-on-glass, silicon-on sapphiresubstrates. The term “substrate” is thus used to define generally theelements for layers that underlie a layer or portions of interest. Also,the “substrate” may be any other base on which a layer is formed, forexample a glass or metal layer. An example of a suitable substrate 1which may be used with preferred embodiments may be a Si wafer.

Onto the substrate 1, optionally a protecting layer 2 and/or a barrierlayer 3 may be provided, e.g. deposited. The protecting layer 2 may, forexample, be a SiO₂ layer, and may, for example, have a thickness of 500nm. The barrier layer 3 may, for example, be a TaN, TiN, TaN/TiN or SiClayer and may have a thickness in the range of, for example, 10 nm to 20nm. The barrier layer 3 may be used to avoid diffusion of metal, in theexample given copper, into the substrate 1. It has to be noted that,according to preferred embodiments, both a protecting layer 2 and abarrier layer 3 may be provided. According to other embodiments, onlyone of a protecting layer 2 or a barrier layer 3 may be provided.According to still other embodiments, none of these layers 2, 3 may beprovided.

In a next step a copper surface layer 4 is provided. The copper layer 4may, for example, be deposited by known deposition techniques such asPlasma Vapor Deposition (PVD) or Electrochemical Deposition (ECD) orelectroless plating. The copper layer 4 may, for example, have athickness in the range of between 200 nm and 1000 nm. However, accordingto preferred embodiments the copper layer 4 may also have a thickness oflarger than 1000 nm. The initial thickness of the copper layer 4, i.e.the thickness of the copper layer 4 after deposition and beforeprocessing according to preferred embodiments is proceeded, may have aninfluence on the length of the Cu-NWs formed later in the process or mayat least have an influence on the copper halide compound formed (seefurther). The structure obtained after deposition of the copper surfacelayer 4 is illustrated in FIG. 1A.

In a next step, the copper surface layer 4 is at least partlyhalogenated. In other words, at least part of the copper surface layer 4is converted into a copper halide. For example, the copper halide may bea copper halide different from a copper fluoride. A problem that canarise with fluoride as halogenide is that the copper fluoride formed isinert with respect to the reaction to form the NWs (see further) becauseit is more stable than other copper halides such as e.g. copper bromideand copper chloride. Converting the copper surface layer 4 into a copperhalide may be done by at least partly exposing the copper surface layer4 to a halogen comprising ambient, such as a halogen comprising plasmaor halogen comprising gas, to form a metal halide (MeH_(x)) surface 5(see FIG. 1B). The halogen comprising plasma or halogen comprising gasmay, for example, be a chlorine or bromine comprising plasma or achlorine or bromine comprising gas to respectively form a CuCl_(x) orCuBr_(x) surface 5. Preferably, when the halogen comprising ambient isformed by a chlorine comprising gas or plasma, it may be a Cl₂-gas or aBCl₃-gas or a Cl₂-plasma. When the halogen comprising ambient is formedby a bromine comprising gas or plasma, it may be a HBr gas or a HBrplasma.

The exposure time period may be in the range of between 1 second and 100seconds, and may, for example, be 10 seconds. An example of an exposureprocess suitable to be used with the method according to preferredembodiments may be performed by exposing at least part of the coppersurface layer 4 to a Cl comprising ambient, such as a Cl₂ plasma with apower of 600 Watt, a pressure of 10 mTorr, without substrate bias andduring an exposure time period of 10 seconds. With power of a plasma ismeant the power at which the plasma is generated. Another example of anexposure process suitable to be used with the method according topreferred embodiments may be performed by exposing at least part of thecopper surface layer 4 to a suitable Br comprising ambient, such as aHBr plasma with a power of 600 Watt, a pressure of 10 mTorr, withoutsubstrate bias and during an exposure time period of 10 seconds.

Instead of a halogen comprising plasma, a halogen comprising gas such ase.g. a bromine and/or chlorine comprising gas may be used. In case ahalogen comprising gas is used to convert the copper surface layer 4into a copper halide surface 5, and a native oxide is present on thecopper surface layer 4, e.g. by long exposure to air, the copper surfacelayer 4 may be cleaned first to remove the native oxide such that aclean copper surface is available for reaction with the halogencomprising gas or plasma.

The substrate 1 comprising the at least partly halogenated copper layer5 is then exposed to photon comprising ambient, e.g. light source or aplasma, to create a photon induced reaction. The photon comprisingambient used needs to provide enough energetic active photons. Thephotons serve to break certain chemical bonds. By breaking these bonds,the growth of the copper comprising nanowire is initiated. Examples ofsuitable photon comprising ambients which may be used according topreferred embodiments may be light sources such as ARC lamps, halogenlamps, fluorescent lamps and suitable plasmas to be used with thepreferred embodiments may be a He plasma, a H plasma or an Ar plasma.Preferably, the photon comprising ambient may be ignited directly abovethe copper halide surface 5. With directly above the copper halidesurface 5 is meant that the photon comprising ambient is in directcontact with the copper halide surface 5. This may be obtained byproviding the substrate with the copper halide surface 5 in the photoncomprising ambient.

Exposure may be performed during a time period of between 30 seconds and300 seconds to induce Cu-NW growth. The time period of exposure may alsobe longer than 300 seconds. However, according to preferred embodiments,it has been observed that an exposure time of longer than 300 secondsdoes not lead to a change in size of the NWs 7 any more. For example,the photon comprising ambient may be a He plasma and exposure may beperformed with a power of 300 Watt for 7 seconds, followed by a power of1000 Watt for 120 seconds, at a pressure in the range of between 20mTorr and 100 mTorr and without substrate bias. Alternatively the photoncomprising ambient may be an Ar or a H plasma. After exposure, Cu-NWs 6are formed on the substrate 1 (see FIG. 1C). In this formation process,the copper halide surface 5 acts as a precursor for initiating thegrowth of Cu-NWs 6. With the copper halide surface 5 acting as aprecursor is meant that the copper halide surface 5 can be used as asubstance to grow the Cu-NWs 6 from.

According to preferred embodiments, during the step of exposing thecopper halide surface layer to a photon comprising ambient, formation ofvolatile copper halide products is initiated. These volatile copperhalide products are responsible for the formation of the Cu-NW or inother words these copper halide products are re-deposited onto thesurface in the form of Cu-NW. The volatile copper halide products may bepartly removed from the reaction chamber during exposure of the copperhalide surface layer to the photon comprising ambient provided that theconcentration of these volatile copper halide products is not below thesaturation level of the volatile copper halide products in the reactionchamber because otherwise no re-deposition of these copper halideproducts to form Cu-NW is possible (prevented).

The minimum allowable concentration of the volatilized copper halideproducts in the reaction chamber (saturation level) can be derived fromequation [1]:

$\begin{matrix}{S = \frac{P_{a}}{P_{e}}} & \lbrack 1\rbrack\end{matrix}$

In which S is the saturation ratio of the gas phase in the reactionchamber, P_(a) is the real partial pressure of the volatilizedhalogenated copper products in the reaction chamber and P_(e) thetheoretical equilibrium partial pressure of volatilized halogenatedcopper products at a given pressure and temperature. The partialpressure of the volatilized halogenated copper products in the reactionchamber can be influenced by the incoming gas flow(s) in the reactionchamber (e.g. He gas flow). The following equation [2] describes therelationship of the incoming gas flow on the actual concentration ofgasses in the reaction chamber at a given pressure and temperature:

$\begin{matrix}{D = {D_{0}\frac{T}{T_{0}}\frac{P_{0}}{P}}} & \lbrack 2\rbrack\end{matrix}$

In which D₀, T₀, P₀ are respectively the gas flow, temperature (25° C.)and pressure (1 atm) of the incoming gas (He) and D, T en P the actualflow, temperature and pressure in the etch chamber. Since the substrateis kept at low temperature special attention has to be paid to theremoval of volatile copper halide products out of the reaction chamber.This requires in general that the incoming gas flow rate is correlatedto the exhaust flow rate.

According to preferred embodiments, the copper halide surface 5 mayfirst be cleaned to remove oxides and/or other contaminants present atthe copper halide surface 5, before the Cu-NW growth is initiated.Cleaning may be done by any suitable cleaning technique known by aperson skilled in the art. For example, when the contaminant is e.g.CuO_(x), a H₂ or N₂ plasmas may be used to remove the contaminants.

The purity of the Cu-NWs 6 depends on the composition of the copperhalide compound which is used as a ‘precursor’. With the methodaccording to preferred embodiments, metal comprising nanowires 6, or ingeneral metal comprising elongated nanostructures, can be formed with adiameter of smaller than 500 nm, for example a diameter of between 50 nmand 200 nm, for example with a diameter of 100 nm, and with a length ofbetween 500 nm and 40 μm, for example with a length of 20 μm or 30 μm.The nanowires 6, or in general the elongated nanostructures, formed bythe method according to preferred embodiments may comprise a combinationof a metal and a metal halide compound. The nanowires may comprisecrystalline, e.g. monocrystalline or polycrystalline, metal and metalhalide compounds. According to preferred embodiments, the nanowires 6,or in general the elongated nanostructures, may comprise between 60% and99% or between 60% and 80% metal and between 1% and 40% or between 1%and 20% metal halide compound. For example, when a CuCl_(x), e.g. CuCl₂layer 5 is formed by exposing a Cu surface layer 4 to e.g. BCl₃ theCu-NWs 6 formed may comprise 95% copper and 5% impurities which may inthe present example mainly be CuCl_(x) (or alternatively, when HBr isused as a plasma, CuBr_(x)). For obtaining pure copper nanowires, or ingeneral pure metal elongated nanostructures, as already described above,the halide compounds can be removed by reducing them to pure copper, orin general to pure metal by exposing them to a H₂ plasma. Conversion ofthe metal halide, e.g. copper halide, into pure metal, e.g. pure copper,then occurs through a simple reduction reaction. According to otherembodiments and as described earlier, metal halide compound elongatednanostructures comprising substantially no pure metal any more may beobtained by exposing the nanowires 6 to a halide.

In the above described example, the substrate 1 may be any suitablesubstrate provided with a copper surface layer 4, or in general a metalsurface layer 4, on top of it. However, according to other preferredembodiments, the substrate 1 may be a bulk copper substrate, or ingeneral, may be a bulk metal substrate.

According to another embodiment, a pattern may be provided onto thecopper surface layer 4 (or copper substrate) before the substrate 1 isexposed to the photon comprising ambient in order to obtain selectivegrowth of Cu-NWs 6. With selective growth of Cu-NWs 6 is meant thatCu-NWs 6 are grown only at predetermined locations on the substrate 1.Again, this embodiment will be described by means of copper nanowires(Cu-NWs). This is only for the ease of explanation and is not intendedto limit the invention in any way. The method according to the presentembodiment can also be applied for, in general, forming metal comprisingelongated nanostructures on predetermined locations on a structure.

FIGS. 2A up to 2E illustrate subsequent processing steps in a methodaccording to preferred embodiments to form Cu-NWs 6 at predeterminedlocations on a substrate 1 comprising at least a copper surface layer 4.The first steps of providing a substrate 1, optionally providing aprotecting layer 2 and/or a barrier layer 3, and providing a coppersurface layer 4 are similar to the steps described in the embodimentabove with respect to FIG. 1A and the structure obtained afterperforming these steps is illustrated in FIG. 2A.

After the steps of providing a substrate 1, depositing a protectinglayer 2 and a barrier layer 3 and depositing a copper surface layer 4, apattern 10 may be provided onto the copper surface layer 4. The pattern10 may be provided on top of the copper surface layer 4 such that someparts of the copper surface layer 4 are exposed and some parts arecovered by the pattern and are thus not exposed. To form the pattern 10onto the copper surface layer 4 an extra layer 7 (see FIG. 2B) may bedeposited and holes or openings 8 may be formed in this layer 7 (seeFIG. 2C). This may be done by means of, for example, a combination oflithographic patterning and etching. The extra layer 7 may comprise adielectric material such as SiO₂, a low-k dielectric layer such asChemically Vapour deposited (CVD) SiCO(H) or an organic spin-on low-klayer such as Silk®. According to preferred embodiments, the pattern 10may be a dummy pattern or, in other words, may be a sacrificial patternthat may be removed after Cu-NWs 6 are formed. Alternatively the pattern10 may be a permanent pattern that will not be removed after Cu-NWs 6are formed. The permanent pattern 10 can then, for example, be used toform interconnect structures in Back-End-of-Line (BEOL) processing of asemiconductor device. FIG. 8 illustrates the presence of a pattern 10with holes or openings 8 on the substrate 1.

In a next step exposed or non-covered parts of the copper surface layer4 are at least partly halogenated by exposing these parts of the coppersurface layer 4 to a halogen comprising ambient, e.g. a chlorine orbromine comprising ambient, as described in the previous embodiment, toform a metal halide (MeH_(x)) compound surface 5, in the example given aCu halide compound such as e.g. CuCl_(x) and/or CuBr_(x) (see FIG. 2D).Optionally, the Cu surface layer 4 may, before exposing to the halogencomprising ambient, first be cleaned to remove possible contaminants. Incase the contaminant is CuO_(x), a H₂ or N₂ plasmas can be used.

In a next step the Cu halide, e.g. CuCl_(x) and/or CuBr_(x), surface 5is exposed to a photon comprising ambient, e.g. light source, toinitiate a photon induced reaction as described above to form Cu-NWs 6into the holes 8 of the pattern 10 such that selective growth of Cu-NWs6 is obtained (see FIG. 2E). According to preferred embodiments, thecopper halide surface 5 may be cleaned before initiating growth ofCu-NWs 6 to remove possibly present contaminants. Optionally, afterformation of the Cu-NWs 6, the pattern 10 may be removed. This may bedone by any suitable technique known by a person skilled in the art.FIG. 9 shows a detail of the part of FIG. 8 illustrated with the dashedsquare. FIG. 9 illustrates the growth of a NW 6 in a hole 8 of thepattern 10.

FIG. 3 schematically illustrates the influence of the initial thicknessof the copper surface layer 4, i.e. the thickness of the copper surfacelayer 4, before exposure to the halogen comprising ambient on theformation of the copper halide compound 5 and on the morphology andlength of the Cu-NWs 6 formed on the copper halide compound 5.

FIGS. 3A up to 3C illustrate formation of Cu-NWs 6 starting from acopper surface layer 4 with initial thickness of approximately 1000 nm(d₁) which results in formation of a Cu halide compound, e.g. CuCl_(x)or CuBr_(x), layer 5 having a thickness (d₂) of between approximately1000 nm and 1500 nm after exposure to a halogen comprising ambient, e.g.to a Cl₂ or HBr plasma during 10 seconds. Due to the transformation ofpart of the copper surface layer 4 to a Cu halide compound, e.g. CuClorCuBr_(x), surface 5, the initial thickness (d₁) of the copper surfacelayer 4 is reduced to a smaller thickness (d_(1′)) (see FIG. 3B). It hasbeen observed that the thickness of the layer 5 of Cu halide compoundformed may be about 5 times higher than the amount of copper that hasbeen used to form the Cu halide compound. For example, in the examplegiven, if the layer 5 of Cu halide compound has a thickness d₂ of 1500nm, about 300 nm of the copper surface layer 4 will have beendisappeared, or in other words, the thickness of the copper surfacelayer 4 may have been reduced with 300 nm. In the example given, thethickness d_(1′)of the copper surface layer 4 may, after formation ofthe copper halide compound be 200 nm. After exposure of the Cu halidecompound, e.g. CuCl_(x) or CuBr_(x), surface 5 to a photon comprisingambient, e.g. light source, to initiate a photon induced reaction asdescribed above, Cu-NWs 6 are formed (see FIG. 3C). The Cu-NWs 6 mayhave a length (d₃) in the range of between 20 μm and 30 μm afterexposure to, for example, a He plasma for 120 seconds.

FIGS. 3D to 3F illustrate formation of Cu-NWs 6 using the same processconditions as described for the process with respect to FIGS. 3A to 3Cbut now starting from a copper surface layer 4 with a lower initialthickness, i.e. with an initial thickness of 500 nm (d₄) (see FIG. 3D)which results in formation of a Cu halide compound, e.g. CuCl_(x) orCuBr_(x), surface 5 with a thickness (d₅) of between approximately 300nm and 500 nm after, for example 10 seconds exposure to a Cl₂ or HBrplasma (see FIG. 3E). Due to the transformation of part of the coppersurface layer 4 to a Cu halide compound, e.g. CuCl_(x) or CuBr_(x),surface 5 the initial thickness (d₄) of the copper surface layer 4 isreduced to a smaller thickness (d_(4′)). In a similar way as describedabove, it has been observed that the thickness d₅ of the layer 5 of Cuhalide compound formed may be about 5 times higher than the amount ofcopper that has been used to form the Cu halide compound. For example,if the layer 5 of Cu halide compound has a thickness (d₅) of 500 nm,about 100 nm of the copper surface layer 4 will have been disappeared,or in other words, the thickness (d_(4′)) of the copper surface layer 4may have been reduced with 100 nm. In the example given, the thickness(d_(4′)) of the copper surface layer 4 may, after formation of thecopper halide compound be 400 nm. After exposure of the Cu halidecompound, e.g. CuCl_(x) or CuBr_(x), surface 5 to a photon comprisingambient, e.g. light source, Cu-NWs 6 are formed with a much smallerlength (d₆) compared to the Cu-NWs 6 formed with the process asillustrated in FIGS. 3A to 3C, i.e. with a thickness (d₆) ofapproximately 10 μm after, for example, 300 seconds exposure to a Heplasma (see FIG. 3F).

From the above it is clear that the initial thickness of the coppersurface layer 4 may determine the thickness of the Cu halide compoundlayer 5 and the length of the Cu-NWs 6 formed. By providing a coppersurface layer 4 with a suitable thickness, nanowires 6, or in generalmetal comprising elongated nanostructures, with a predetermined lengthmay be formed by using the method according to preferred embodiments.

FIG. 4A to 4C illustrate the influence of pressure in the plasma chamberduring exposure of a CuCl_(x) layer 5 to a He plasma for nanowiregrowth. The upper drawings show a cross-section and the lower drawingsshow top views of the substrate 1 with Cu-NWs 6 grown on top. FIG. 4Aillustrates a SEM picture for Cu-NWs 6 grown by exposing to a He plasmaat 7 mTorr. As can be seen from the figure, this results in ratherisolated, free-standing Cu-NWs 6. FIG. 4B illustrates a SEM picture forCu-NWs 6 grown by exposing to a He plasma at 30 mTorr. This results inmassive growth of Cu-NWs 6. With massive growth is meant that a hugeamount of nanowires is present at the substrate 1 and that neighbouringnanowires are entangled with each other. FIG. 4C illustrates a SEMpicture for Cu-NWs 6 grown by exposing to a He plasma at 80 mTorr. Ascan be seen, this does not result in formation of Cu-NWs 6, but information of hillocks.

The above described experiment illustrates that pressure during exposureto a photon comprising ambient may be an important parameter in order toobtain Cu-NWs with desired properties. By tuning the pressure in theplasma chamber during growth of the nanowires 6, free-standingnanowires, or in general free-standing elongated nanostructures, may beformed with the method according to preferred embodiments. Specialattention must be paid that during exposure to the photon comprisingambient, the concentration of the volatile copper halide products in thereaction chamber needs to be above the saturation level of the volatilecopper halide products to initiate formation of at least one metalcomprising elongated nanostructure.

FIGS. 5A and 5B illustrate the influence of plasma power during exposureof the Cu halide layer 5 to a He plasma on nanowire growth. The upperdrawings show a cross-section and the lower drawings show top views ofthe substrate 1 with Cu-NWs 6 on top. FIG. 5A illustrates a SEM picturefor Cu-NWs 6 grown by exposing to a He plasma at a power of 300 Watt fora time period of 600 seconds. As can be seen, this results in isolated,free-standing Cu-NWs 6. FIG. 4B illustrates a SEM picture by exposing aHe plasma at a power of 1000 Watt for 600 seconds. This results inmassive growth of Cu-NWs 6 and a high coverage of the surface of thesubstrate 1.

Hence, from the above described experiment it can be seen that by tuningthe power in the plasma chamber during nanowire growth, free-standingCu-NWs 6, or in general free-standing metal comprising elongatednanostructures may be formed by the method according to preferredembodiments.

Depending on process parameters such as e.g. pressure, temperature,halide source, the NWs 6 formed may have different shapes. FIG. 10illustrates Cu NWs 6 grown starting from a CuCl_(x) layer formed ofexposing Cu to a Cl₂ ambient. FIG. 11 illustrates Cu NWs 6 grownstarting from a CuBr_(x) layer formed of exposing Cu to a HBr ambient.It can be seen that in case of FIG. 10 the NWs 6 have a ‘curled’ shapewhile in case of FIG. 11 the NWs 6 have a straight shape. It has to beunderstood that also curled shapes can be obtained for Cu NWs 6 startingfrom a CuBr_(x) layer and that straight shapes can be obtained for CuNWs 6 starting from a CuCl_(x) layer when process parameters arechanged.

Methods according to preferred embodiments will hereinafter beillustrated by some experiments. It has to be understood that theseexperiments are only illustrative and are not intended to limit theinvention in any way.

Experiment 1: Formation of Cu-NWs Starting From a CuCl_(x) Layer byExposure to a He Plasma

A copper layer 4 was first deposited on a Si substrate 1. A barrierlayer 3 was deposited prior to deposition of the copper surface layer 4.The barrier layer 3 may, for example, be a SiO₂ layer. The thickness ofthe copper layer 4 may be in the range of between 500 nm and 1000 nm.First, the copper surface layer 4 was exposed to a Cl₂ plasma to convertthe copper layer 4 into a CuCl_(x) layer 5. The process parameters usedto perform this step are summarized in Table 1. The experiments wereconducted in a Lam Versys 2300 etch chamber.

TABLE 1 Working example of process conditions using a Lam Versys 2300etch chamber for performing the step of exposing a Cu layer to a Cl₂ gasto form a CuCl_(x) layer. parameter Setting Power in plasma chamber 600Watt Pressure in plasma chamber 10 mTorr Exposure time 10 secondsSubstrate bias no

In the next step, the CuCl_(x) layer 5 was exposed to a He plasma toinitiate the growth of Cu-NWs 6. The process parameters to perform thisstep are summarized in Table 2. The experiments were conducted in a LamVersys 2300 etch chamber.

TABLE 2 Working example of process conditions using a Lam Versys 2300etch chamber for performing the step of exposing a CuCl_(x) layer to aHe plasma to form Cu-NWs. parameter Setting Power in plasma chamber 1000Watt (*) Pressure of He plasma 30 mTorr Exposure time 120 secondsSubstrate bias no (*) starting with 7 seconds at 300 Watt

Experiment 2: Formation of Cu-NWs Starting From a CuBr_(x) Layer byExposure to a He Plasma

A copper layer 4 is first deposited on a Si substrate 1. A barrier layer3 was deposited prior to deposition of the copper surface layer 4. Thebarrier layer 3 may, for example, be a SiO₂ layer. The thickness of thecopper layer 4 may be in the range of between 500 nm and 1000 nm. First,the copper surface layer 4 was exposed to a HBr plasma to convert thecopper layer 4 into a CuBr_(x) layer 5. The process parameters used toperform this step are summarized in Table 3. Experiments were conductedin a Lam Versys 2300 etch chamber.

TABLE 3 Working example of process conditions using a Lam Versys 2300etch chamber for performing the step of exposing a Cu layer to a HBr gasto form a CuBr_(x) layer. parameter Setting Power in plasma chamber 600Watt Pressure in plasma chamber 10 mTorr Exposure time 10 secondsSubstrate bias no

Next, the CuBr_(x) layer 5 was exposed to a He plasma to initiate thegrowth of Cu-NWs 6. The process parameters used to perform this stepwere the same as described for the first example and are summarized inTable 2 above. Experiments were conducted in a Lam Versys 2300 etchchamber.

Experiment 3: Auger Electron Spectroscopy (AES) Experiments toCharacterize the Cu-NWs Formed by the Method According to PreferredEmbodiments

By using AES the composition and chemistry of a surface can be examinedby measuring the energy of electrons emitted from that surface when itis irradiated with electrons having an energy in the range of between 2keV and 50 keV. During the experiments, it was tried to focus the AESbeam on top of the Cu-NWs 6 formed by the method according to preferredembodiments. The spot size for AES is about 1 μm² which resulted in goodfocusing on the Cu-NWs 6. The penetration depth of the AES beam is lessthan 10 nm so there could be differentiated between the nanowires 6 andthe copper halide surface 5.

In case the Cu surface 4 was converted into a CuCl_(x) surface using aCl₂ comprising gas and the photon induced reaction was initiated using aHe plasma (see example 1) both the Cu-NWs 6 formed and the CuCl_(x)surface 5 next to the Cu-NWs 6 comprise Cu, Cl and O. FIG. 6 illustratesthe AES survey scan recorded on one Cu-NW 6 starting from a CuCl_(x)layer 5 and on the surface underneath the Cu-NW 6. Both curves arecoinciding and cannot be distinguished very well in the figure. The Clconcentration (indicated with reference number 20) in the Cu-NWs 6 seemsto be lower than the Cl concentration on the CuCl_(x) surface (indicatedwith reference number 21). Oxygen is present because samples were olderthan a few days and oxidation had occurred.

In case the Cu surface 4 was converted into a CuBr_(x) surface 5 using aHBr comprising gas and the photon induced reaction was initiated using aHe plasma (see example 2) both the Cu-NWs 6 formed and the CuBr_(x)surface 5 next to the Cu-NWs formed comprise Cu, Br and O. FIG. 7Aillustrates the AES survey scan recorded on one Cu-NW 6 starting fromthe CuBr_(x) layer and FIG. 7B illustrates the AES survey scan recordedon the substrate surface underneath the Cu-NWs 6. The Br concentrationin the Cu-NWs 6 seems to be lower than the Br concentration on theCuBrsurface 5. Oxygen is present because samples were older than a fewdays.

In case the Cu surface 4 was transferred into a CuCl_(x) surface 5 usinga BCl₃ comprising gas and the photon induced reaction was created usinga He plasma both the Cu-NWs 6 formed and the CuCl_(x) surface 5 next tothe Cu-NWs 6 formed comprise Cu, Cl and O. The Cl concentration in theCu-NWs 6 seems to be lower than on the CuCl_(x) surface. Oxygen ispresent because samples were older than a few days and oxidation hadoccurred.

Experiment 4: Ion Beam Analysis (IBA) Experiments to Characterize theCu-NWs Formed by the Method According to Preferred Embodiments

Ion beam analysis is an analytical technique involving the use of ionbeams with an energy in the order of MeV to examine the composition ofand to obtain elemental depth profiles in single crystals. Measurementswere performed at Cu-NWs 6 which were fabricated starting from a Cusurface layer 4 which was converted into a CuCl_(x) surface 5 using aBCl₃ comprising gas and the photon induced reaction was initiated usinga He plasma. In this experiment, 17 MeV iodine ions were bombarded ontothe Cu-NWs 6. The Cu-NWs 6 were inspected after analysis and seemed tobe intact so only the nanowires 6 are probed and not the surface. Thiscan be concluded because the NWs 6 form a dense structure through whichthe iodine ions cannot penetrate unless the NWs 6 are destroyed by theseiodine ions. Table 5 shows the composition of Cu-NWs 6 determined byusing Ion Beam Analysis according to this experiment.

TABLE 5 Composition of Cu—NW using Ion Bean Analysis in which the Cu—NWare bombarded with 17 MeV iodine ions. compound % H 13.4 Cl 8.5 C 1.6 O29.8 Cu 46.8

Experiment 5: GI-XRD Experiments to Characterize the Cu-NWs Formed bythe Method According to Preferred Embodiments

In this experiment, measurements were performed on CuCl_(x) and CuBr_(x)surfaces 5 without Cu-NWs 6 to find out which peaks on the samplescomprising nanowires 6 really originate from the presence of thesenanowires 6.

From comparison of the presence of a CuCl_(x) peak between samples withand without Cu-NWs 6 and after oxidation (in clean room (CR)environment) it is concluded that diffraction peaks in the spectra ofthe samples with Cu-NWs 6 originate only from the Cu-NWs 6 themselves.

In case the Cu surface 4 was converted into a CuCl_(x) surface using aCl₂ comprising gas and the photon induced reaction was initiated using aHe plasma the Cu-NWs 6 formed comprise both crystalline CuCl_(x) andcrystalline Cu. After oxidation in the CR environment CuCl_(x)disappeared and was replaced with probably CuO or Cu₂O.

In case the Cu surface 4 was transferred into a CuBr_(x) surface 5 usinga HBr comprising gas and the photon induced reaction was initiated usinga He plasma the Cu-NWs 6 formed comprise both crystalline CuBr_(x) andcrystalline Cu. The Cu-NWs 6 seemed to resist well to oxidation becauseafter a few days almost no other peaks appear in the spectrum.

In case the Cu surface 4 was transferred into a CuBr_(x) surface 5 usinga BCl₃ comprising gas and the photon induced reaction was initiatedusing a He plasma the Cu-NWs 6 formed comprise both crystalline CuBr_(x)and crystalline Cu. After oxidation in the CR environment CuBr_(x)disappeared and was replaced with probably CuO or Cu₂O. It has to benoted that in this case a very intense Cu peak was observed in thespectrum which indicates a large amount of crystalline Cu present in theCu-NWs 6.

Experiment 6: EDX (Energy Dispersive X-ray Analysis) Experiments toCharacterize the Cu-NWs Formed by the Method According to PreferredEmbodiments.

Energy dispersive X-ray spectroscopy (EDS or EDX) is an analytical toolused for chemical characterization. Being a type of spectroscopy, itrelies on the investigation of a sample through interactions betweenlight and material. The characterization capabilities of this techniqueare in large part due to the fundamental principle that each element ofthe periodic table has a unique electronic structure and thus a uniqueresponse to electromagnetic waves. The spot size for probing wasapproximately 1 μm³. The samples were inspected in cross section todifferentiate between substrate 1 and Cu-NWs 6.

In case the Cu surface 4 was converted into a CuCl_(x) surface 5 using aCl₂ comprising gas and the photon induced reaction was initiated using aHe plasma the Cu-NWs 6 formed were found to comprise 20% CuCl_(x) and80% Cu.

In case the Cu surface 4 was converted into a CuBr_(x) surface 5 using aHBr comprising gas and the photon induced reaction was initiated using aHe plasma the Cu-NWs 6 formed were found to comprise 20% CuBr_(x) and80% Cu.

In case the Cu surface 4 was transferred into a CuBr_(x) surface 5 usinga BCl₃ comprising gas and the photon induced reaction was initiatedusing a He plasma the Cu-NWs 6 formed were found to comprise 5% CuBr_(x)and 95% Cu.

The above described experiments show that with the method according topreferred embodiments it is possible to form metal comprising elongatednanostructures comprising a high percentage of metal. By tuningparameters of the process, metal comprising elongated nanostructures 6with predetermined properties may be obtained.

It is to be understood that although preferred embodiments, specificconstructions and configurations, as well as materials, have beendiscussed herein for devices according to the preferred embodiments,various changes or modifications in form and detail may be made withoutdeparting from the scope of this invention as defined by the appendedclaims. For example, steps may be added or deleted to methods describedwithin the scope of the preferred embodiments.

All references cited herein are incorporated herein by reference intheir entirety. To the extent publications and patents or patentapplications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe specification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained by thepreferred embodiments. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of theclaims, each numerical parameter should be construed in light of thenumber of significant digits and ordinary rounding approaches.

1. A method for forming in a reaction chamber at least one metalcomprising elongated nanostructure onto a substrate, the methodcomprising: converting at least part of a metal surface layer of asubstrate into a metal halide compound; exposing the metal halidecompound to a photon comprising ambient to initiate formation of atleast one metal comprising elongated nanostructure; and during exposureto the photon comprising ambient, volatile metal halide products areformed and the concentration of the volatile metal halide products inthe reaction chamber is above the saturation level of the volatile metalhalide products to initiate formation of at least one metal comprisingelongated nanostructure.
 2. The method of claim 1, wherein the metal iscopper and the elongated nanostructure is a copper comprising elongatednanostructure, wherein the substrate comprises at least a copper surfacelayer and wherein converting at least part of the metal surface layerinto a metal halide compound is performed by converting at least part ofthe copper surface layer into a copper halide compound.
 3. The method ofclaim 1, wherein converting at least part of the metal surface layerinto a metal halide compound is performed by exposing at least part ofthe metal surface layer to a gas comprising halogen.
 4. The method ofclaim 1, wherein converting at least part of the metal surface layerinto a metal halide compound is performed by exposing at least part ofthe metal surface layer to a plasma comprising halogen.
 5. The method ofclaim 3, wherein exposing at least part of the metal surface layer to aplasma comprising halogen is performed at about 600 Watt and about 10mTorr during an exposure time period of about 10 seconds.
 6. The methodof claim 3, wherein the halogen is selected from the group consisting ofHBr or Cl₂.
 7. The method of claim 1, wherein exposing the metal halidecompound to a photon comprising ambient is performed by exposing themetal halide compound to a plasma comprising at least one gas selectedfrom the group consisting of He, Ar, and H.
 8. The method of claim 7,wherein the metal halide compound is CuCl_(x) and the photon comprisingambient is a He plasma, wherein exposing the metal halide compound to aphoton comprising ambient is performed at about 1000 Watt and about 30mTorr without substrate bias during an exposure time period of at leastabout 30 seconds.
 9. The method of claim 7, wherein the metal halidecompound is CuBr_(x) and the photon comprising ambient is a He plasma,wherein exposing the metal halide compound to a photon comprisingambient is performed at about 1000 Watt and about 80 mTorr withoutsubstrate bias during an exposure time period of at least about 30seconds.
 10. The method of claim 7, wherein the exposure time period isabout 120 seconds.
 11. The method of claim 1, further comprising a stepof providing a metal surface layer onto a substrate, wherein the step isperformed before the step of converting at least part of a metal surfacelayer of a substrate into a metal halide compound.
 12. The method ofclaim 11, further comprising, before providing a metal surface layeronto the substrate, a step of providing at least one of a protectivelayer and a barrier layer onto the substrate.
 13. The method of claim 1,further comprising, before converting at least part of the metal surfacelayer into a metal halide compound, a step of providing a pattern ontothe metal surface layer.
 14. The method of claim 13, wherein providing apattern onto the metal surface layer is performed by: providing a layerof material onto the metal surface layer; and providing at least onehole in the layer of material.
 15. The method of claim 14, whereinproviding a layer of material is performed by providing a layer ofdielectric material.
 16. The method of claim 13, further comprising,after formation of the metal comprising elongated nanostructure, a stepof removing the pattern.
 17. The method of claim 1, further comprising,before exposing the metal halide compound to a photon comprisingambient, a step of cleaning the substrate.
 18. The method of claim 17,wherein cleaning the substrate is performed by a H₂ or N₂ plasma. 19.Use of the method of claim 1 in a method for forming copper comprisingelongated nanostructures.
 20. A metal comprising elongated nanostructurecomprising a combination of a metal and a metal halide compound, whereinthe metal forms a core of the elongated nanostructure and the metalhalide compound forms a shell of the elongated nanostructure, the shellsubstantially completely surrounding the core.
 21. The metal comprisingelongated nanostructure of claim 20, wherein the metal comprisingelongated nanostructure comprises from about 60% to about 99% metal andfrom about 1% to about 40% metal halide.
 22. The metal comprisingelongated nanostructure of claim 21, wherein the metal comprisingelongated nanostructure comprises more than about 95% metal and lessthan about 5% metal halide.
 23. The metal comprising elongatednanostructure of claim 20, the metal comprising elongated nanostructurehaving a length and a diameter, wherein the length of the metalcomprising elongated nanostructure is from about 500 nm to about 40 μmand wherein the diameter of the metal comprising elongated nanostructureis from about 50 nm to about 200 nm.
 24. The metal comprising elongatednanostructure of claim 20, wherein the metal is copper and the metalhalide is a copper halide.
 25. The metal comprising elongatednanostructure of claim 24, wherein the copper halide is CuCl_(x) orCuBr_(x).
 26. The metal comprising elongated nanostructure of claim 20,wherein the metal comprising elongated nanostructure is a metalcomprising nanowire.